In recent years, a submarine cable service has been developed rapidly. But sometimes, due to a special location, a submarine cable is hard to maintain and manage directly. However, the submarine cable plays an important role in a transport network, which requires that an equipment vendor provides a corresponding product for monitoring a submarine equipment. A monitoring device needs to monitor an entire submarine cable system without affecting the submarine fiber service, provide information required by a maintenance engineer, and perform trouble location, monitoring, and management, which helps to maintain the submarine cable system.
Currently, a submarine cable system is constructed by a form of distributing fiber pairs. To facilitate locating of a submarine cable fault, fiber pairs are connected by a form of coupling. Probe light undergoes Rayleigh scattering and/or Fresnel reflection in the fiber. A part of scattered light and/or reflected light is returned to a probe device through a loopback path. The state of the submarine cable line is monitored by analyzing the scattered light and/or reflected light.
FIG. 1 is a simplified block diagram of a submarine cable system. The submarine cable line termination A is located on one end (end A) of the submarine cable system, and submarine cable line termination B is located on the other end (end B) of the submarine cable system. Termination A and termination B are connected through submarine cables (such as L1, L2, and L3) to transmit the submarine cable service. A repeater is used to amplify optical signals transmitted on the fiber in the submarine cable. Fiber L11 and fiber L12 are a fiber pair in submarine cable L1. Using terminal A as a reference point, a signal sent from end A to end B is an uplink signal, and a signal received on end A from end B is a downlink signal. The fiber that bears the uplink signal is an uplink fiber, such as L11 and L21 in FIG. 1; and the fiber that bears the downlink signal is a downlink fiber, such as L12 and L22 in FIG. 1. FIG. 1 is only a visualized description of a submarine cable system. In a practical submarine cable system, the distance between end A and end B is generally hundreds or even thousands of kilometers, the number of repeaters between end A and end B is far more than two. Monitoring unit A and monitoring device B are configured to monitor fiber lines in the submarine cable. A probe light sent by the monitoring unit A combines with a service light sent by the submarine cable line termination A, and is transmitted to a peer end after passing through the submarine optical cables and the submarine repeater. The submarine cable fiber pair is looped back and coupled inside the repeater so that the probe light generated by the monitoring unit A may return to the monitoring unit A through the loopback path in the repeater. In this way, a probe signal is received and analyzed, and the submarine cable system is monitored.
In the conventional art, an out-to-out loopback mode is applied to monitor the state of the submarine line, as shown in FIG. 2. The out-to-out loopback mode means that in the repeater, the output end of an optical amplifier (OA) 1 is connected with a coupler 1, and the output end of an OA 2 is connected with a coupler 2; the coupler 1 is connected with the coupler 2 through a fiber, and the path formed by the fiber, the coupler 1, and the coupler 2 is called an out-to-out loopback path. In this loopback mode, the method for monitoring the submarine cable line is: The monitoring device A sends probe light 1, the probe light 1 passes through the OA1 and the coupler 1 in the uplink fiber, and continues to be transmitted in the uplink fiber of the submarine cable between the repeater 1 and the repeater 2; in this uplink fiber, the probe light 1 undergoes Rayleigh scattering and/or Fresnel reflection, and therefore, a part of the probe light 1 is transmitted reversely in the uplink fiber, and this part of light is transmitted to the coupler 1 and coupled to the output end of the OA2 of the downlink fiber along the out-to-out loopback path, and finally, returned to the monitoring device A along the downlink fiber. By detecting the probe light scattered and/or reflected, the monitoring device A analyzes and monitors the state of the uplink fiber and locates the fault of the uplink fiber. The method for locating the fault of the downlink fiber is the same as the method for locating the fault of the uplink fiber except that the monitoring device B on the peer end sends probe light 2 and that the scattered and/or reflected probe light of the monitoring device B are transmitted back through the uplink fiber and are finally received and detected.
In the research process, the inventor of the present invention finds that the out-to-out loopback mode in the conventional art for monitoring the submarine cable line has at least the following two problems: a monitoring device on one end of the submarine cable system is capable of monitoring the state of one fiber only (an uplink fiber or a downlink fiber), and is unable to monitor both the state of the uplink fiber and the state of the downlink fiber.